RNA interference (RNAi) refers to post-transcriptional gene silencing mediated by small double stranded RNAs. Hundreds of non-coding RNAs, called microRNAs, are transcribed from numerous genomes ranging from worms to humans. MicroRNAs are highly conserved and regulate the expression of genes by binding to the 3′-untranslated regions (3′-UTR) of specific mRNAs. Several cellular processing steps produce biologically active, 19-25 nucleotide RNA fragments that, together with a group of proteins called the RNA Induced Silencing Complex (RISC), mediate gene silencing in a sequence-specific fashion. Importantly, endogenous microRNA machinery can be appropriated; vector delivered short hairpin RNAs (shRNAs) can enter the RNAi pathway and induce silencing of any gene of interest.